Once-through heater



FIG. 1 is a perspective view of a once-through heater showing my new design;

FIG. 2 is a top-plan view thereof, it being understood that the orientation is arbitrarily chosen;

FIG. 3 is a right-side elevational view thereof;

FIG. 4 is a left-side elevational view thereof;

FIG. 5 is a front elevational view thereof;

FIG. 6 is a rear elevational view thereof;

FIG. 7 is a bottom plan view thereof;

FIG. 8 is a view similar to FIG. 1 of a second embodiment of the invention; and,

FIG. 9 is a view similar to FIG. 2 of the second embodiment, it being understood that all further views of the second embodiment would be identical to FIGS. 3-7. 

The ornamental design for a once-through heater, as shown and described. 